Astronomers spot the first cloud-free exoplanet

An important arm of exoplanet research involves the study of the atmospheres that surround these distant worlds. The molecules they hold can reveal valuable insights into what kind conditions can be found there – along with ones that cannot. Such is the case for scientists studying WASP-96b, who are now claiming to have discovered the first exoplanet to be free of clouds.

Described as a "hot Saturn," WASP-96b circles a Sun-like star around 980 light years away in the constellation of Pheonix. Astronomers at the University of Exeter and Dublin City University were observing the planet with the Very Large Telescope in Chile, studying its atmosphere as it passed in front of its host star.

This technique is known as the transit method and involves detecting the dimming of a host star by an exoplanet passing in front of it. NASA's Kepler Space Telescope has discovered the presence of thousands of exoplanets using this method, but by looking at what kind of light that makes its way through, scientists can also draw conclusions about a planet's atmosphere.

This is because an atmosphere will block certain wavelengths of light depending on the kinds of molecules and atoms within it, creating a unique spectrum for that planet. And it just so happens that the unique signature of WASP-96b contains the complete fingerprint for sodium, a chemical element that produces a tent-shaped spectra and can only be seen in atmospheres without clouds.

"Until now, sodium was revealed either as a very narrow peak or found to be completely missing," says Nikolay Nikolov, lead author from the University of Exeter. "This is because the characteristic 'tent-shaped' profile can only be produced deep in the atmosphere of the planet and for most planets clouds appear to get in the way."

The team was also able to measure the abundance of sodium in the atmosphere of WASP-96b, observing levels similar to those found in our own Solar System. Its presence suggests that the exoplanet WASP-96b has an atmosphere free of clouds, which not only furthers our understanding of the universe, but also clears the way for further inspection.

"WASP-96b will also provide us with a unique opportunity to determine the abundances of other molecules, such as water, carbon monoxide and carbon dioxide with future observations," says co-author Ernst de Mooij, from Dublin City University.